Music disappeared from the iPhone

It happened to me twice in the last couple of months: all music suddenly disappeared from my iPhone, except for tracks purchased from the iTunes store.

Sync with iTunes on my Mac also seems to be broken: It shows that all missing songs are on the iPhone and running sync does not change anything.

Any ideas what might cause this behaviour? I did a full restore after it happened the first time, but obviously it did not fix the issue. It's an iPhone 6 running the latest version of iOS.

Thanks